We provide a system and method for reverse osmosis treatment of water, including seawater and brackish water. Methods and systems of embodiments of the invention may include, for example, ultrafiltration followed by biofoulant removal, both of which precede reverse osmosis. In preferred embodiments the system is run at a low flux. For example, a flux of 6-8 GFD may be used with seawater. Additional embodiments may provide the above process in conjunction with a reverse-osmosis membrane cleaning system. The membrane cleaning system is a “clean in place” system that includes use of the natural pressure differential in the reverse osmosis system to remove biofoulants and their precursors.